1. Field of the Invention
The present invention relates to a chipless thread-forming screw and, in particular, to a thread-forming screw for screwing metal sheets and having a thread-carrying shank, a tip provided at one end of the shank, and a head provided at another end of the shank. The shank has at least one frustroconical section extending from the tip in direction of the head, and a cylindrical section extending between the frustroconical section and the head.
2. Description of the Prior Art
U.S. Pat. No. 4,194,430 discloses a thread-forming screw having a head and a shank that adjoins the head. The shank passes at its end remote from the head in a frustroconical section and ends in a tip or a punctiform tip. The thread surrounds the shank, with the thread height continuously tapering from the end region of the shank up to the tip.
The drawback of such thread-forming screw consists in that it can form a thread in a hard material only in a pre-drilled bore. When the screw is used for not pre-drilled sheet metal, then it can primarily be used only for very thin sheets, such as used, e.g., in the motor vehicle industry. For screwing a stack with a plurality of sheets lying one above another, this screw is not suitable.
U.S. Pat. No. 5,597,357 discloses a self-drilling screw which is provided at an end of the screw shank remote from the head with a drilling tip. With this self-drilling screw, it is possible to penetrate sheet metal having a thickness of more than 2 mm.
The drawback of the above-described screw consists in that during a setting process, chips are produced which should be removed, for corrosion reasons, with great costs. Further, during a setting process, a user should apply a high press-on force during the entire process. In addition, drilling leads to loss of material which adversely affects the holding force of the screw.
Accordingly, an object of the present invention is to provide a screw in which the above-discussed drawbacks are prevented.
Another object of the present invention is to provide a thread-forming screw which would be able to chiplessly penetrate a stack of at least two metal sheets arranged one above another with a total thickness of the sheets of at least 2 mm.